1. Field of the Invention
This invention relates to a side-light type backlight, including a light source which includes LEDs and a light guide plate, and a liquid crystal display device.
2. Description of the Related Art
Backlights used conventionally for a liquid crystal display device can be classified into a side-light type backlight in which a light source is arranged on an end surface side of a light guide plate and a just-below type backlight in which a light source is arranged just below a liquid crystal panel. The side-light type backlight has mainly been used for liquid crystal display devices having a size of about 20 inches or below, particularly for liquid crystal display devices requiring a reduced thickness.
In either system, a cold cathode-ray tube has been used for the light source. LEDs are being used, at present, for compact display sizes such as portable cellular telephones and PDAs because much light power is not required and LEDs are suitable for reducing the size and the weight.
The cold cathode-ray tube is predominant in the liquid crystal display devices having a greater display size than those of the cellular telephones and the PDA. However, the problem of protecting the environment has been taken seriously and the use of the cold cathode-ray tube, which uses mercury, is not desirable. Therefore, various light sources such as mercury-free florescent tubes, LEDs, and so forth have been developed to replace the cold cathode-ray tube. Among them, LEDs are very promising as a light source for next generation devices.
The side light type backlight including the LEDs is described in Japanese Unexamined Patent Publications (Kokai) No. 2001-174816 and No. 2002-350846, for example. The former patent reference 1 teaches to arrange red, blue and green rod-like light sources along a periphery of a light guide plate. The latter patent reference 2 teaches to use four colors of LEDs, that is, white, red, blue and green LEDs.
When the LEDs are used as the light source in the side light type backlight, there may be the case where a plurality of white LEDs are arranged along one or a plurality of sides of a light guide plate and the case where a plurality of red LEDs (R-LEDs), a plurality of green LEDs (G-LEDs) and a plurality of blue LEDs (B-LEDs) are arranged along one or a plurality of sides of the light guide plate to create the white color. The following problems occur at this time.
(a) To secure the amount of light in the light guide plate and uniformity of chromaticity. When a plurality of LEDs are arranged along one side of the light guide plate, these LEDs are arranged with a certain interval among them. Therefore, regions where light is absent appear at portions close to the light incidence surface and, as the distance increases, light mixes with light from the neighboring LEDs, and light becomes uniform at a position away from the light incidence surface.
(b) Drop of light emission efficiency due to heat generation of the LEDs and variation among the LEDs. The LEDs are preferably cooled because light emission efficiency drops with heat generation. Light emission efficiency is different among the R-LEDs, the G-LEDs and the B-LEDs depending on the temperature, and the R-LEDs are most susceptible to the influences of the temperature. Unless a LED is satisfactorily cooled, non-uniformity of the amount of light and non-uniformity of color occur.
(c) To secure reliability. When the LEDs are used at a certain temperature for a long time, light emission efficiency drops. The drop of light emission efficiency is also different among the R-LEDs, the G-LEDs and the B-LEDs. Therefore, there is the case where non-uniformity of the amount of light and non-uniformity of colors appear after the use for thousands of hours. There is the possibility that deterioration of any LEDs among a plurality of LEDs results in non-uniformity of the amount of light and non-uniformity of colors when a plurality of LEDs are used.
(d) To improve efficiency. To obtain the amount of light equivalent to that of the cold cathode-ray tube by using the LEDs, a large number of LEDs are necessary. This results in the increase in the cost and consumed power. Therefore, a plurality of LEDs must be utilized efficiently.